Mobile communication terminals have developed such that they are capable of more than just a simple wireless communication function. For example, a camera function and a multimedia replaying function can be provided. The camera function and the multimedia replaying function have gained popularity simultaneously with developments in display unit technology for the mobile communication terminal and high-speed communication techniques. Specific developments include high-resolution digital cameras and mobile terminals having color LCD screens.
Streaming video/audio data decoded by an MPEG-4 codec (coder and decoder) or video/audio data inputted by a camera may be displayed on the color LCD screen of the mobile terminal. Currently, a mobile communication terminal capable of real-time transmission and reception of video/audio data inputted by a camera for video telephony is being actively researched.
Because the mobile communication terminal receives video data through a video real time protocol (RTP) session and receives audio data through an audio RTP session, the received video/audio data must be synchronized with each other. The transmission of video/audio data of a mobile communication terminal in accordance with the related art will be explained with reference to FIG. 1.
FIG. 1 illustrates a method for transmitting video/audio data of a mobile communication terminal in accordance with the related art. As shown in FIG. 1, a mobile communication terminal sets a video/audio RTP session and transmits a video/audio RTP packet through the video/audio RTP session. Also, the mobile communication terminal sets a video/audio RTP control protocol (RTCP) session for controlling the video/audio RTP packet, and transmits a video/audio RTCP packet through the RTCP session. The video/audio RTCP packet denotes a video/audio RTCP sender report (SR) packet.
A method for synchronizing video/audio data of a mobile communication terminal in accordance with the related art will be explained with reference to FIG. 2. As shown in FIG. 2, a method for synchronizing video/audio data of a mobile communication terminal comprises receiving video/audio data through a set session (S210), calculating a standard time based on the received video/audio data (S220), and synchronizing the received video/audio data based on the calculated standard time (S230).
The method for synchronizing video/audio data of a mobile communication terminal in accordance with the related art will be explained in more detail.
The mobile communication terminal receives a video RTP packet through a video/audio RTP session and receives a video RTCP packet through a video/audio RTCP session. Also, the mobile communication terminal receives an audio RTP packet through the video/audio RTP session and receives an audio RTCP packet through the video/audio RTCP session (S210). A method for transmitting the received RTP packet and the RTCP packet will be explained with reference to FIG. 3.
FIG. 3 is a block diagram illustrating the receipt of an RTP packet and an RTCP packet in accordance with the related art. As shown in FIG. 3, the mobile communication terminal periodically receives the RTCP packet for controlling the RTP packet, and calculates a standard time of the received RTP packet. The standard time provides information on absolute time since an RTP time stamp is a relative time that can be used for a single RTP session. Also, a network time protocol (NTP) time stamp may be used to calculate the standard time. Accordingly, the standard time is used for synchronizing the video/audio data based on the RTP time stamp and the NTP time stamp of the (i+1)th RTCP packet and the previously received ith RTCP packet.
The mobile communication terminal calculates a standard time for synchronizing the received video/audio data based on the received video/audio RTCP packet by the following formula (1) (S220).
                              NTP          time_stamp                =                              NTP                          time_stamp              ,                              i                +                1                                              +                                                    Δ                ⁢                                                                  ⁢                                  NTP                  time_stamp                                                            Δ                ⁢                                                                  ⁢                                  RTP                  time_stamp                                                      ⁢                                          (                                                      RTP                    time_stamp                                    -                                      RTP                                          time_stamp                      ,                                              i                        +                        1                                                                                            )                            .                                                          (        1        )            
The RTPtime—stamp, i, and RTPtime—stamp, i+1 respectively denote an RTP time stamp included in the ith RTCP packet and an RTP time stamp included in the (i+1)th RTCP packet. The NTPtime—stamp, i, and NTPtime—stamp, i+1 respectively denote an NTP time stamp included in the ith RTCP packet and an NTP time stamp included in the (i+1)th RTCP packet.
A variation value of the network time protocol (NTP) time stamp and a variation value of the real time protocol (RTP) time stamp are calculated by the following formulas (2) and (3).ΔNTPtime—stamp=NTPtime—stamp, i+1−NTPtime—stamp, i  (2).ΔRTPtime—stamp=RTPtime—stamp, i+1−RTPtime—stamp, i  (3).
The mobile communication terminal can synchronize the received video/audio data based on the calculated standard time (S230).
However, in the method for synchronizing video/audio data of a mobile communication terminal in accordance with the related art, a standard time has to be calculated whenever video/audio data is received. Accordingly, even if the variation value of the NTP time stamp and the variation value of the RTP time stamp are constant, the time for synchronizing the video/audio data is not shortened.